This research is designed to provide information on how the brain of infrahuman primates becomes assembled as a means of understanding normal and pathological development of the human brain and improving prevention and treatment of congenital brain diseases. The prenatal development of the central nervous system (CNS) in rhesus monkeys is studied by a combination of several advanced neurobiological methods of fetal neurosurgery. Analysis of each structure or system proceeds in three phases. In the first phase attention is given to the interactions of brain cells as they multiply and migrate and as postmigratory neurons develop processes and establish synaptic connections that ultimately constitute the "wiring" arrangements of the mature brain. In the second phase, the consequences of selective destruction of various brain centers and/or pathways performed on the fetus are evaluated at postnatal ages to determine the extent of neuronal plasticity and the interdependence of various structures. Finally, in order to relate experimental results obtained from studies in monkeys to humans, cytologically defined developmental stages are correlated in both species. Projects currently under study concern: 1. the normal development of the auditory system, pyramidal motor system and its synaptic targets, extrapyramidal system and monoamine system; 2. Pathological development and neuronal plasticity in the auditory system, pyramidal and cortico-ponto-cerebellar system and 3. Correlation of "critical periods" in the developing monkey and human brain.